dark terrain is needed to match the patches’ reflectance (12). Subsequent images covering the Anhur
region were acquired on 4 to 5 June, revealing
that the water ice had fully sublimated from the
surface, leaving a layer spectrally indistinguishable
from the average nucleus (Fig. 2).

We compute the sublimation rate for this period by applying the thermal model described in
(18) for both intimate and areal mixtures of refractory material and ice (12). The estimated ice
loss rate ranged from 1.4 to 2.5 kg day−1 m−2 for
the intimate mixture and from 0.14 to 0.38 kg
day−1 m−2 for the areal mixture cases. By noting
that the permanence of the ice patches was about
10 days, we estimate a solid-ice equivalent thickness
between 1.5 and 27 mm for the ice patches. The
actual thickness of this layer will be up to a factor
of 10 higher due to porosity of the dust/ice mixture.

The appearance and disappearance of water-ice patches shows that the level of activity isvarying on time scales that are short comparedwith seasonal changes of the illumination. Theseice-rich patches indicate a variation of the water-ice content in the uppermost layers, pointing tolocal compositional heterogeneities with scalesof 10s of meters on 67P’s nucleus. The huge width-to-depth ratio observed in the ice patches wouldsuggest a near-surface solar-driven process beingresponsible for enhancement of local ice abun-dance, resulting from the recondensation of vol-atiles and sintering of the subsurface materialduring previous perihelion passages. Laboratoryresults from Kometen Simulation (KOSI) exper-iments had shown that a considerable fractionof sublimating ice can be redeposited insteadof being released through the dust mantle (20).Numerical simulations show that a hardened layermay form beneath a fine dust mantle (21), a hardlayer that was detected by the Philae lander in theAbydos site (22). The composition of the icy patchesmay be representative of the comet’s near surface.Occasional local removal of the overlying mantl-ing material could expose the underlying layer, lead-ing to an icy surface for limited periods of time.Approaching perihelion, the nucleus has shownconsiderable diurnal color variations on extendedareas and the occurrence of water frost close tomorning shadows. This is evident on the Imhotepregion, where morphological changes were ob-served (23). Areas just emerging from the shad-ows are spectrally bluer than their surroundings(Fig. 3A), while 40 min later, once fully illuminated,their spectral slope has increased (Fig. 3B). Thisphenomenon, observed during other color se-quences acquired in June and July 2015, and seenat dawn on different areas on both lobes of thecomet (12) (fig. S5 and movie S1), is periodic. Weinterpret the relatively blue surface at dawn asthe presence of additional water frost that con-densed during the previous night.We also observe the presence of fronts of brightmaterial in the illuminated regions close to rapid-ly traveling shadows cast by local topography(Fig. 3). A water-rich fringe near shadows at theHapi/Hathor transition was also observed byVIRTIS (7). The bright fronts move with theshadows. Modeling of the illumination (fig. S6and movie S2) shows that the extent of these brightfeatures directly correlates with the shadow travelspeed, being wider where the shadow speed isfaster and narrower where the shadow speed isslower.These fronts are about six times as bright asthe mean comet reflectance. Their spectrum isglobally flat with a flux enhancement in the ul-traviolet (Fig. 4 and fig. S7), similar to that ob-served on blue regions of comet Tempel 1 (1). Anabundance of about 17% water frost linearly mixedwith the comet dark terrain is needed to matchthe reflectance of the bright fronts (12). We inter-pret these bright features as surface frost, formedwhen water vapor released from the subsurfacerecondenses after sunset, that rapidly sublimateswhen exposed to the Sun. Molecules in the inner

480 nm filters) showing
the appearance of the
bright patches in the
Anhur/Bes regions (A to

D), and associated zooms
(E to H); the arrows indicate two common
boulders. The reflectance
relative to 535 nm and
the normal albedo are
represented in (I) and (J).

The black line represents
the mean spectrum of
the comet from a region
close to the patches.

Dashed and dotted lines
(J) show the best-fit
spectral models to the
patches (associated
uncertainty shown in
gray) produced by the
linear mixture of the
comet dark terrain (12)
enriched with 21 ± 3% of
water ice (dashed line) or

23 ± 3% of water frost
(dotted line) for patch B,
and with 29 ± 3% of water
ice (dashed line) or 32 ±
3% of water frost (dotted
line) for patch A.